1. Field of the Invention
The present invention relates to an encoding/decoding apparatus. Image information is quantized, and then encoded. A decoder then decodes the encoded image information.
2. Related Background Art
For the purpose of encoding a color image signal, the ADCT (Adaptive Discrete Cosine Transformation) method of encoding is gaining popularity. This method is based on encoding by decomposing the image into its spatial frequency components, and is one of the known orthogonal transformation encoding methods.
The ADCT encoding is also adopted by the JPEG (Joint Photographic Expert Group) established as an international standard organization for such encoding methods, and the ADCT is gaining popularity as the orthogonal transformation most suitable for encoding image information.
The encoding and decoding by ADCT are in general conducted in the following manner.
The transformation of two-dimensional frequency components is executed in a unit block of 8 .times.8 pixels, since such transformation for the entire picture requires an excessively long time.
The ADCT is conducted for each block to obtain ADCT coefficients in an 8.times.8 matrix, in which the coefficients at the right side, lower side and lower right area respectively signify the high frequency components in the horizontal, vertical and diagonal directions, and the coefficient at the upper left corner indicates the DC component.
Then said ADCT coefficients are quantized, rougher in the higher frequency side and finer in the lower frequency side, with weighting on the quantizing levels for the ADCT coefficients based on the human visual characteristics.
Then thus quantized ADCT coefficients are encoded. Since the components are concentrated in the lower frequency side, the encoding is conducted by zigzag scanning between the high and low frequency components. The significant coefficients with non-zero quantized outputs are grouped according to their values, and Haffman encoding is executed on a set of an identification number for the group, an immediately preceding significant coefficient, and a number positioned therebetween and indicating the run length of insignificant coefficients with zero quantized outputs, followed by the addition of a same-length code identifying the value in the group.
Since the diagonal high frequency components generally have low probability in an image, the latter portion after zigzag scanning is expected to become all zero in most cases. Consequently, a very high compression rate can be expected in the variable length code obtained in this manner, and an image reproduction with minimal deterioration in image quality can be achieved if a compression rate of several to one is anticipated in average.
However, the transmission channel generally has a fixed transmission capacity per unit time, and, if a picture has to be transmitted within a predetermined period as in the case of transmitting a moving image, it is desirable that the obtained code has a fixed number of bits (transfer rate) for a picture or for a unit pixel block.
The number of bits per picture is determined by a control coefficient C, which determines the coarseness of quantization. Thus, the control coefficient C is an important parameter for determining the transfer rate of an image and the quality thereof.
Various methods have been proposed for determining the control coefficient C.
For each frame of image data compressed as explained above, the control coefficient C employed in the quantization of image data is attached, and the inverse quantization is conducted with the control coefficient C at the reproduction of the compressed image data, whereby the image is reproduced through a process exactly inverse to the encoding process.
Thus, in the conventional art, the information for control coefficient C for quantization is attached to the image information of each frame.
The coefficient information, being an extremely important parameter for determining the quantization characteristics of the image and thus influencing the image quality, may destruct the quantization properties of an image frame and provide a visually unpleasant image if an error is involved in the information, for example in the transmission channel.